The present invention relates to an optical data transmission system and, more particularly, to an optical receiver applicable to an optical local area network which interconnects a number of optical terminals by optical fiber to permit them to transmit data therebetween.
Recently, the art of optical fiber communication has made rapid progress and found applications in various fields. Typical of the applications is a proposed optical local area network which effects transmission of data among a large number of optical terminals.
Advantages of such a network are that it is flexible and economical. Other advantages are that the use of nonconductive optical transmission lines safeguards the system against troubles due to electromagnetic interference and the like, and that the reliability of the network is excellent because the terminals, which is not either transmitting or receiving need not be in operation.
A randomly accessed local area network relies on distributed control and asynchronous packet transmission. Communication control for this type of system is usually implemented by a system in which each terminal monitors the presence/absence of a signal from another terminal and, if present, restrains transmission therefrom (see, for example, U.S. Pat. No. 4,063,220). This system, called Carrier Sense Multiple Access with Collision Detection (CSMA/CD), has to be constructed such that one transmitting terminal the presence/absence of a signal from another terminal as the presence/absence of signal collision. So long as a transmission line employed is of the type using coaxial cables or the like for communication by electric signals, signals from all the terminals will be received at a substantially common level and, therefore, signal collision will be found out with ease.
When it comes to light, however, the transmission loss in the optical fibers and optical couplers is so significant that the power received differs a great deal from one terminal to another. If the difference in power is insignificant, even an ordinary optical receiver made up of a photodetector, an amplifier and a decision circuit will successfully detect signal collision by monitoring the variation in received signal power (see, for example, an article entitled "Fibernet II: An Active Star-Configurated Fiber-Optic Local Computer Network with Data Collision Sensing" by Eric G. Rawson et al., Fifth Topical Meeting on Optical Fiber Communication, Apr. 13-15, 1982, OSA/IEEE, Phoenix, Ariz.). However, should the difference increase beyond a certain level, an intense optical signal would hide a less intense optical signal making the signal collision undetectable.